Microsoft has launched a campaign in the U.S. that targets Google's alleged practice of going through the contents of all Gmail messages to sell and target advertisements, raising again an old issue the Redmond, Washington, software giant has with the free email service.

The company late Wednesday cited a study which showed that 70 percent of consumers don't know that major email providers routinely engage in the practice of reading through their personal email to sell ads.

88 percent of people disapprove of the practice once they are informed, according to a GfK Roper study commissioned by Microsoft.

Unlike Gmail, Microsoft's Outlook.com doesn't go through the content of users' emails to show ads, it said.

The "Don't Get Scroogled by Gmail" campaign on Microsoft's Scroogled.com promotes Outlook.com as an alternative to Gmail. Microsoft previewed Outlook.com in July and said it would eventually replace its Hotmail email service.

Advertising keeps Google and many of the websites and services the company offers free of charge, Google said in an emailed statement on Wednesday. "No humans read your email or Google Account information in order to show you advertisements or related information. An automated algorithm -- similar to that used for features like Priority Inbox or spam filtering -- determines which ads are shown," it added.

Outlook.com is also calling on consumers to join a petition drive on Scroogled.com to tell Google to stop going through their users' email to sell ads, Microsoft said.

Microsoft sponsored Don't Get Scroogled activities will appear online and offline, demonstrating why consumers should be concerned, and helping them take action, Microsoft said. "This campaign is as much about protecting Outlook.com users from Gmail as it is about making sure Gmail users know what Google's doing," said Stefan Weitz, senior director of online services at Microsoft, in a statement.

The company has been critical of a settlement in January between the U.S. Federal Trade Commission and Google over the search giant's business practices and access to its standards-essential patents, calling it a "missed opportunity."

John Ribeiro covers outsourcing and general technology breaking news from India for The IDG News Service. Follow John on Twitter at @Johnribeiro. John's email address is john_ribeiro@idg.com.